1. Field of the Invention
The present invention relates to a cylinder-injection type two cycle combustion engine where fuel is directly injected into a combustion chamber.
2. Description of the Related Art
A conventional cylinder-injection type two cycle combustion engine is provided with a cylinder, a piston reciprocating within the cylinder, a fuel injector for directly injecting fuel into a combustion chamber, a spark plug for igniting mixture of fuel injected into the combustion chamber and air, an exhaust port provided at the cylinder so as to perform exhaust when the piston is moved downward, a plurality of scavenging paths provided along a circumferential direction in a spaced manner for scavenging the combustion chamber when the piston is moved downward, and a crank shaft which is transmitted with upward and downward movements of the piston to be rotated. In this case, the cylinder is formed with a cylinder body formed in a cylindrical shape with its proximal end opened towards a crank chamber, a liner provided in a cylindrical manner so as to cover an inner surface of the cylinder body, and a cylinder head closing a distal end opening of such a cylinder body. The fuel injector and the spark plug are assembled to the cylinder head. The fuel injector is structured so as to directly inject fuel towards a head portion of the piston. A recessed portion is provided at a position on a head portion of the piston against which fuel from the fuel injector directly strikes or in the vicinity thereof, so that the fuel injected from the fuel injector is reflected at the recessed portion towards an inner surface of the cylinder head. The respective scavenging paths are provided along and between the cylinder body and the liner positioned on an inner surface thereof, and their outlet ports are opened to the liner. Inlet ports of the scavenging paths are respectively opened to the crank chamber. Incidentally, there is a case where a liner is not provided on an inner surface of a cylinder body. In such a case, respective scavenging paths are formed in the block of the cylinder body along the axial direction thereof.
In such a conventional cylinder-injection type two cycle combustion engine, the top portion of the piston is formed in a generally horizontal surface or flat surface so as to facilitate injection of scavenging air from the respective scavenging paths into the cylinder at a scavenging time.
In such a conventional cylinder-injection type two cycle combustion engine, however, since the top portion of the piston is made generally flat except for the recessed portion, scavenging air merely flows along the top portion, so that cooling for the piston becomes insufficient at a scavenging time. As a result, there has been a drawback that the temperature of the top portion of the piston becomes high, and in some cases, the piston seizes the liner or the inner surface of the cylinder.
In order to prevent the piston from seizing, there is a method where a clearance between the liner or the inner surface of the cylinder body and the outer peripheral surface of the piston is secured excessively. In such a case, however, there has been a drawback that such a demerit as reduction in engine output or power, increase in piston striking noises or the like is caused.
An object of the present invention is to provide a cylinder-injection type two cycle combustion engine where seizure of a piston can be prevented without excessively forming a clearance between a liner or an inner surface of a cylinder and an outer surface of a piston.
Another object of the invention is to provide a cylinder-injection type two cycle combustion engine where cooling a piston can efficiently be performed at least in the vicinity of an exhaust port in which seizure of a piston is easy to occur.